1. Field of the Invention
This invention relates to a liquid crystal display (LCD) element, and more particularly relates to an active matrix type liquid crystal display element having a driving circuit unit and pixel unit formed combinedly that is capable of accepting a digital signal having a signal level lower than a power source voltage level of a horizontal driving circuit system.
2. Description of Related Art
Recently, the trend that LCD monitors separated from notebook type personal computers (referred to as personal computer hereinafter) being used as desktop type monitors has grown in response to the development of thin LCD monitors of reduced power consumption. The internal circuit of a personal computer is structured so that digital signals are processed. On the other hand, a CRT monitor is driven by analog signals, therefore the input output I/F (interface) is an analog I/F. However, because a LCD itself of a-Si uses mainly a source driver IC of a digital I/F, A/D conversion should be performed again somewhere, such conversion is very inefficient as a total system.
As for the state of the art of the driving circuit combined type LCD, while a sampling system of an analog video signal as shown in FIG. 5 is developed, a circuit having a digital I/F has not been realized. Herein, the system in accordance with the conventional example shown in FIG. 5 is described. Between a signal line 101 for transmission of a analog video signal and column lines 102-1 and 102-n, n transfer gates 103-1 to 103-n are connected.
These transfer gates 103-a to 103-n are turned on (becomes conductive) at the rising edge of sampling pulses .phi.1, .phi.2, . . . , .phi.n supplied successively from the H shift register 104 to sample an analog video signal, and supplies successively it to column lines 102-1 to 102-n. On the other hand, m row lines 105-1 to 105-m are driven successively by the V shift register 106.
On respective intersection points of n column lines 102-1 to 102-n and m row lines 105-1 to 105-m, a thin film transistor (TFT) is provided. A source electrode of the thin film transistor 107 is connected to a column line 102-1 to 102-n, a gate electrode is connected to a row line 105-1 to 105-n respectively. A drain electrode of the thin film transistor 107 is connected to the transparent pixel electrode of pixels respectively arranged two dimensionally in the form of a matrix.
The system in accordance with the conventional example having the structure described herein above is advantageous to a small sized LCD of, for example, the view finder of a video camera or the light bulb of a projector in that a full color (full analog) display is realized with a relatively simple structure. However, application to a large sized or medium sized LCD results in a significant disadvantage.
(1) Use of a large sized LCD panel inevitably leads to use of large capacity video line and source line (column line), and a large power is consumed when signals are charged/discharged rapidly. Further, an analog buffer for driving such load results in very large EMI (Electromagnetic Interference) source, and set design is difficult.
(2) It is considered in order to cope with the problem (1) that an analog signal is divided into a multiplicity of divided signals and divided analog signals are supplied, however it is very difficult to eliminate the dispersion between channels of a multiplicity of divided analog signals. Further, the system will be a very complex and large system.
(3) Point-successive sampling timing and phase control of video signals are very difficult and the image quality inevitably becomes poor due to ghost.
For the reason described herein above, a large sized driving circuit combined LCD has not been realized up to today. In the field of a-Si (amorphous silicon) LCD, heretofore a method in which a silicone LSI is mounted near a panel using mounting method of TAB (Tape Automated Bonding) and a signal is supplied is employed. However, cost of silicon LSI and mounting cost of a silicon LSI results directly in the increased panel cost.